In a gas turbine, during the running time, its power and efficiency decrease due to contamination, deposits, erosion and corrosion, with the result that the overall power station process is adversely influenced. Especially the aerodynamic parts of the compressor at the inlet of the gas turbine are in this case affected.
The contamination of the gas turbine is caused by the adhesion of particles to the surfaces. Oil and water mists contribute to the possibility of dust and aerosols settling on the blades. The contaminations and deposits occurring most frequently are mixtures of wettings with water and water-soluble and water-insoluble materials. In the gas turbine, contaminations due to ash deposits and unburnt solid cleaning preparations may occur. Such air pollutants adhere in the manner of scales to the components of the flow path of the gas turbine and react with them. Further, grazings due to the impact of particles and to abrasion occur and are generally designated as erosion.
Furthermore, ice fragments which form at the inlet of the gas turbine may come loose and strike the components in the flow path of the gas turbine. In order to prevent this, an anti-icing system, as it is known, is employed. Air preheating, here, prevents the situation where the temperature of the air upon entry into the gas turbine does not fall below the freezing point and therefore the water does not freeze.
Owing to the ageing processes described, increased surface roughness of the blades is caused. This leads to comparatively high frictional losses in the gas turbine, since laminar boundary layer flows may change over to a turbulent flow, thus resulting in growing flow resistance. Furthermore, gaps in the gas turbine increase in size due to abrasion and corrosion. The losses caused by the increased gap flow rise, and the performance of the plant decreases.
The influence of ageing phenomena is especially high at the inlet of the gas turbine, which is the compressor. Geometric variations in the blades due to erosion, deposits and damage bring about a reduced performance of the gas turbine. Deposits, erosion and corrosion occurring at the inlet lead to modified inlet angles which have a very pronounced effect on the thermodynamic performance. An aged compressor may sometimes lead to flow stalling.
The aging of the compressor has an adverse effect on the gas turbine efficiency, gas turbine power output and gas turbine outlet mass flow. In order to counteract the reduction in power of the turbine plant, regular compressor scrubs are carried out. Compressor blades may in this case be scrubbed in the online and offline mode. In the online mode, the turbine plant continues to operate during cleaning, and the gas turbine load is lowered only slightly. Online scrubs are employed mainly to avoid the build-up of the dirt layer. An online scrub is usually carried out once a day with fully demineralized water and every third day with cleaning agents.
By contrast, for an offline scrub, the plant is shut down. In order to avoid thermal stresses, it is cooled for six hours with the aid of a shaft rotation device. An offline scrub is usually carried out about once a month. If the turbine plant has not been cleaned for a comparatively long period of time, an offline scrub has to be carried out, as a rule, for typical plants, since the method of online cleaning can no longer remove the dirt.
An offline scrub in this case brings about a greater recovery of power than an online scrub. With the aid of an offline scrub, power recoveries of several percent can be achieved. An online scrub brings about a lower power recovery. The most effective blade cleaning can be obtained by means of a combination of online and offline scrubs. A regular online scrub extends the time intervals between the required offline scrubs.
The optimal time point for an offline scrub is often determined by the operator according to purely economic operating factors, for example in off-peak periods. This means that the decision on the time point for eliminating a contamination of one of the components of the turbine plant, for example by means of a scrub of the compressor, is based solely on empirical values from economic standpoints or from preliminary studies with fixed boundary conditions.
Alternatively, the determination of the time point of the offline scrub may take place on the basis of a current prediction of the power gain of the gas turbine expected as a result of the offline scrub. In this case, such a prediction is usually made on the basis of the development of the compressor efficiency of the gas turbine which serves as a characteristic quantity for the intensity of contamination of the compressor. Such predictive methods are known, for example, from WO 2005/090764 A or from Schepers et al.: “Optimierung der Online- and Offline-Wäsche an einer 26-MW-Gasturbine unter besonderer Berücksichtigung der Leistungssteigerung”[“Optimization of the online and offline scrub on a 26-MW gas turbine, taking particular account of the power increase”], VGB Kraftwerkstechnik, Vol. 79 No. 3.
However, the measurement data used for determining the compressor efficiency may have comparatively high data uncertainties, thus making it more difficult to conduct an exact prediction of the power gain expected as a result of an offline scrub and, consequently, a determination of the time point, cost-optimal for operating the gas turbine, for such an offline scrub.
To increase the accuracy of a prediction of this kind, the statistical uncertainties should in this case be minimized. This may take place, for example, by means of an improvement in the measurement apparatus or an increase in the number of measurements. In this case, however, such an increase only leads to a reduction in the statistical error, but systematic errors in the prediction of the additional power should also largely be minimized. This can be achieved by additionally adopting further characteristic quantities for predicting the additional power. Such a quantity which is characteristic of the power of the gas turbine is the suction mass flow of the gas turbine.
The suction mass flow as a characteristic quantity for the operating power of the gas turbine is usually not measured directly on account of the high outlay, the high measurement uncertainty and the risk of damage, but, instead, is determined indirectly by means of assessments. For a direct measurement, highly complicated instruments would have to be used, since, firstly, there are very high temperatures and, secondly, it is absolutely essential to prevent the sensors from breaking off because of the probably high consequential damage to the turbine blading.